1. Field of the Invention
The present invention relates to a process for preparing acrolein or acrylic acid or a mixture thereof from propane, by    A) feeding to a first reaction zone A at least two gaseous, propane-containing feed streams, at least one of which comprises fresh propane, and, in reaction zone A, subjecting their propane fed in this way to a heterogeneously catalyzed dehydrogenation to obtain a product gas mixture A comprising propane and propylene,    B) conducting product gas mixture A out of reaction zone A and, in a first separation zone A, removing at least a portion of the constituents, other than propane and propylene, present in product gas mixture A, and using remaining product gas mixture A′ comprising propane and propylene    C) in a second reaction zone B to charge at least one oxidation reactor and, in the at least one oxidation reactor, subjecting the propylene present in product gas mixture A′ to a (selective) heterogeneously catalyzed gas phase partial oxidation with molecular oxygen to give a product gas mixture B comprising acrolein or acrylic acid or a mixture thereof as the target product and also excess molecular oxygen,    D) conducting product gas mixture B out of reaction zone B and, in a second separation zone B, removing target product present in product gas mixture B, and, of the remaining residual gas comprising unconverted propane, molecular oxygen and any unconverted propylene, recycling at least a portion comprising unconverted propane, molecular oxygen and any unconverted propylene as one of the at least two propane-containing feed streams into reaction zone A.
2. Description of the Background
Acrylic acid is an important basic chemical whose uses include as a monomer for preparing polymers which are employed, for example, dispersed in aqueous medium, as a binder. Acrolein is an important intermediate, for example for the preparation of glutaraldehyde, methionine, folic acid and acrylic acid.
The process, described in the preamble of this document, for preparing acrolein or acrylic acid or a mixture thereof from propane is known (for example from WO 01/96270, US 2003/0187299 A1 and from the documents DE-A 10245585 and DE-A 10246119 and the prior art cited in these documents).
In the processes for removing target product present in product gas mixture B, the target product is transferred from the gaseous into the condensed phase, for example, by absorptive and/or condensative measures. Useful absorbents include, for example, water, aqueous solution and/or organic solvent. In the course of this “condensation” of the target product, a residual gas which is not transferred into the condensed phase normally remains and comprises the constituents of product gas mixture B which are comparatively difficult to condense. These are typically in particular those components whose boiling point at atmospheric pressure (1 bar) is ≦−30° C. (their total content in the residual gas is generally ≧70% by volume, frequently ≧80% by volume and in many cases ≧90% by volume). These include primarily unconverted propane, excess molecular oxygen remaining in reaction zone B and any unconverted propylene. In addition, the residual gas will generally contain inert diluent gases, e.g. N2, CO2, noble gases (He, Ne, Ar etc.), CO and also, to a small extent, acrylic acid, acrolein and/or H2O (the steam content in the residual gas may be up to 25% by volume, frequently up to 20% by volume or up to 10% by volume, but in many cases also below 10% by volume or below 5% by volume). This aforementioned residual gas forms (based on the amount of propane contained therein) the majority (normally at least 80%, or at least 90%, or at least 95%, or more) of the residual gas formed in separation zone B and is therefore also referred to in this document, inter alia, as main residual gas.
Especially when the condensation of the target product is effected by absorption by means of an organic solvent, at least one second residual gas comprising unconverted propane and any unconverted propylene is generally obtained in separation zone B (based on propane contained therein, its amount in comparison to the amount of main residual gas is normally substantially smaller). This can be attributed to the fact that, as the condensed phase forms, it also absorbs to a certain extent unconverted propane and any unconverted propylene. In the further course of the extractive, distillative, crystallizative and/or desorptive removal of the target product from the condensed phase, this unconverted propane and any propylene is normally recovered as a constituent of at least one further gas phase and preferably recycled into reaction zone A. This may be effected, for example, in a mixture with the main residual gas (in that case, referred to in this document as overall residual gas). However, it may also be effected in the form of independent gas streams to be recycled into reaction zone A. The latter may be oxygen-free or else oxygen-containing (secondary residual gas) (for example, when it is obtained by stripping by means of air or at the top of a rectification column flushed by means of air as a polymerization inhibitor).
In the context of this invention, main residual gas, overall residual gas and secondary residual gas are all residual gas comprising unconverted propane, molecular oxygen and any unconverted propylene which can be recycled into reaction zone A. According to the invention, molecular oxygen-free residual gas which is obtained in separation zone B and comprises unconverted propane and any unconverted propylene may be recycled into reaction zone A in a mixture with main residual gas and/or secondary residual gas (i.e., for example, as a constituent of overall residual gas) and/or else independently (in this case, it is residual gas which is not recycled into reaction zone A in the context of the invention). In the latter case, this recycling may be effected without any restriction, i.e., for example, even as a constituent of the starting reaction gas mixture of reaction zone A. Preference is given in the process according to the invention to recycling the entire amount of the gas streams comprising the unconverted propane and any unconverted propylene obtained in separation zone B into reaction zone A. Portions may (as will be explained in detail in the further course of the application), if appropriate, also be used for other purposes, for example for energy generation and/or synthesis gas production and/or as a diluent gas in reaction zone B.
In the aforementioned prior art documents, the recycling of residual gas comprising unconverted propane, molecular oxygen and any unconverted propylene into reaction zone A should be at the same point as the feed of the remaining propane-containing feed streams into reaction zone A (i.e. as the constituent of the starting reaction gas mixture; cf., for example, FIG. 6 in US-2003/0187299 A1). A disadvantage of this procedure is that the molecular oxygen present in the residual gas reduces the selectivity of propene formation in reaction zone A and increases the selectivity of by-product formation of the carbon oxides CO and CO2 as a consequence of partial full combustion of propane and/or propylene. This is especially true when the residual gas to be recycled is main residual gas.
DE-A 10211275 attempts to remedy the aforementioned problem by proceeding as described above but simultaneously dividing the product gas mixture A formed in reaction zone A into two portions of identical composition and recycling one of the two portions into reaction zone A as the hydrogen source. However, a disadvantage is that the deactivation rate of the dehydrogenation catalysts in this procedure is not fully satisfactory.